Charge transfer bond length method

When charge-transfer bonds are obscured by those of the original donors and acceptors, one may find of value a difference method [1] (e.g., Forster s tandem method [70]). Four cuvets of equal path length are used, two containing the charge-transfer complex solutions in series in the indicator beam of a double-beam spectrophotometer, and two cuvets, one with the unreacted donor and the other with unreacted acceptor solution, also in series in the reference beam. A difference spectrum is thus obtained which, however, needs special care in its interpretation. 

As mentioned above and discussed in Chapter 2, atomic charges were often obtained in the past from dipole moments of diatomic molecules, assuming that the measured dipole moment equal to the bond length times the atomic charge. This method assumes that the molecular electron density is composed of spherically symmetric electron density distributions, each centered on its own nucleus. That is, the dipole moment is assumed to be due only to the charge transfer moment Mct. and the atomic dipoles Malom are ignored. 

But it is also true that the present use of transferable bond energy parameters, such as those found in Eqs. (A.47)-(A.69), translate in a conceptually simple way the logics associated with shifts of rr-electron centroids, with bond lengths, and with the appropriate charge redistributions. The results surely illustrate the flexibility of the method capable of generating the parameters adapted to the various situations that are encountered. Of course, this demands thought. 

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